This invention relates to tomosynthesis and, more particularly, to a method and apparatus that utilizes a tomosynthesis system to acquire 2-D projection images of an object and which then reconstructs a 3-D representation of the object by utilizing a cone beam volumetric computed tomography reconstruction algorithm.
Circular tomosynthesis enables a three dimensional (3-D) image of an object to be constructed from a finite set of two-dimensional (2-D) projection radiograph images. FIG. 1 illustrates the system geometry of a typical circular tomosynthesis system. The system comprises an x-ray source 1 and a 2-D x-ray detector 2, which is normally a film screen. During data acquisition, both the x-ray source 1 and the detector 2 rotate in circular trajectories and a set of projection radiographs of the object 3 are acquired by the detector 2 at discrete source locations. The circular trajectories are in planes that are parallel to each other and the motions of the source 1 and detector 2 are in opposite directions along the trajectories.
Once the projection radiographs have been obtained, they are then digitized and spatially translated with respect to each other and superimposed in such a manner that the images of structures in the tomosynthesis plane overlap exactly. The images of structures outside the tomosynthesis plane do not overlap exactly, resulting in a depth-dependent blurring of these structures. By varying the amount of relative translation of the projection radiographs, the location of the tomosynthesis plane can be varied. Each time the tomosynthesis plane is varied, the image data corresponding to the overlapping structures is superimposed and a 2-D image of the structure in the tomosynthesis plane is obtained. Once a complete set of 2-D images of the object has been obtained, a 3-D image of the object is generated from the set of 2-D images.
Circular tomosynthesis can be very useful for clinical applications that require high spatial resolution in the direction of coronal slice through the patient, especially since it is usually difficult to obtain high resolution in the coronal slice direction with x-ray computerized tomography (CT). Moreover, the apparatus for tomosynthesis is usually simple and inexpensive compared to x-ray CT. However, one shortcoming of circular tomosynthesis systems is that the image quality of the reconstructed 3-D images usually is limited due to blurring of the structures outside of the tomosynthesis plane. Many reconstruction algorithms have been developed to improve the image quality and to reduce this out-of-plane blurring. For example, algorithms based on matrix inversion techniques have been utilized for this purpose.
A well known technique for generating 3-D reconstructions using 2-D projection images is known as cone beam volumetric computed tomography (cone beam VCT). FIG. 2 illustrates the system geometry of a cone beam VCT system. The x-ray source 4 projects a cone of x-rays onto the object 5. The x-rays impinge on a digital detector 6 which digitizes the data. The detector 6 is in a plane which is not parallel to the plane in which the source 4 is located. Rather, the detector 6 is located in a plane that is orthogonal to the plane in which the source is located.
As with circular tomosynthesis, relative motion is generated between the detector 6/source 4 geometry and the object 5. This can be accomplished by rotating the source 4 and the detector 6 or by rotating the object 5. Images of the object are acquired at certain locations of the source 4 and the detector 6 or at certain locations of the object 5, depending on the manner in which the relative motion is generated. As 2-D maps of the image intensity measured by the detector 6 are generated, these 2-D maps are filtered using known image filtering techniques and are then reconstructed using one or more known algorithms to obtain a 3-D representation of the object.
The 3-D images reconstructed using cone beam VCT systems have higher image quality than the reconstructed images obtained using typical circular tomosynthesis systems. It would be desirable to improve the quality of images obtained using circular tomosynthesis systems, rather than replacing circular tomosynthesis systems with cone beam VCT systems. Replacing circular tomosynthesis systems with cone beam VCT systems may not be an option in many cases due to costs and other factors.
Accordingly, a need exists for a method and apparatus which improves the quality of images reconstructed from image data acquired using circular tomosynthesis systems.